Industrial gas turbines often require complex control systems for efficiently converting energy while minimizing polluting emissions. Pollutants such as nitrogen oxide may be reduced by lowering the maximum gas temperature, which may be achieved by maintaining a lean fuel-to-air ratio in the combustion chamber. However, if the fuel/air mixture is too lean, incomplete fuel combustion can produce excessive carbon monoxide and unburned hydrocarbons. Other operational problems emerge when operating with lean combustion, including unstable load transitions and combustion instabilities. Therefore, the fuel/air mixture and the temperature in the reaction zone must be controlled to support complete combustion.
Systems have been proposed for controlling the fuel/air mixture by measuring various combustion parameters, and by using the measurements as input to control the fuel system. For example, one conventional system includes a control system where fuel flow rates, pressure levels, and discharge exhaust temperature distributions are utilized as input for setting fuel trim control valves.
Other techniques for controlling combustion dynamics include measuring light emission from the combustion burner flame, and using the measured signal to control certain combustion parameters. For example, one conventional system uses a closed loop feedback system employing a silicon carbide photodiode to sense the combustion flame temperature via the measurement of ultraviolet radiation intensity. The sensed ultraviolet radiation is utilized to control the fuel/air ratio of the fuel mixture to keep the temperature of the flame below a predetermined level associated with a desired low level of nitrogen oxides.
Other conventional systems can use optical fibers for gathering and transmitting light from a combustion region to detectors. Yet other conventional systems can use a video camera to capture images of the flame primarily for monitoring the presence or absence of a flame.
Mass flux sensing techniques have been proposed for use in turbines. For example, laser-based Doppler-shift measurement systems may be used for determining airflow in a turbine air-inlet duct, and similar systems have been proposed for measuring the static temperature by comparing the absorption features from two light generators (lasers) of different frequency. A need remains for improved systems and methods for closed loop emission control.